Why Fathers Have Lower Levels of Testosterone

Fathers may not want to hear that their testosterone levels are dropping, but new research bears that out, finding that the male hormone plummets in men after they have children — most dramatically in dads who really involve themselves in the nitty-gritty of child rearing.

The findings suggest that women aren’t the only ones who are biologically geared for raising children. Plunging testosterone may help a man commit to his family and reinforce his focus on caregiving, amping up his sensitivity to his children’s needs. “If this weren’t something that had been normative in humans for the last 100,000 or more years, there would be no reason to expect this decline in testosterone,” says lead author Lee Gettler, a biological anthropologist and doctoral candidate in Northwestern’s Department of Anthropology.

This isn’t the first time that declines in testosterone have been linked to fatherhood, but previous research involved small samples observed during brief “snapshots” in time, making it impossible to divine the answer to this conundrum: does fatherhood itself cause testosterone to plummet, or are men with lower testosterone more likely to become fathers?

Researchers at Northwestern University set out to collect data over time to shed some light on the issue. They retrieved testosterone via saliva samples from 465 Filipino men when they were 21 and single, then repeated the same tests when the men were 26. The men — who gave samples first thing in the morning, when testosterone levels are highest, and again in the evening, after they’d declined — were part of a 28-year project that has followed the same cohort since they were in the womb.

In the five years between study segments, some of the men remained single, others found partners but remained childless, while still others had gone on to have children. The single men experienced a modest decline in testosterone (the hormone naturally declines with age), while the fathers notched a dramatic nosedive — twice as much as their single counterparts, according to the research, which was published Monday in the Proceedings of the National Academy of Sciences (PNAS). The men who had partners but no kids fell somewhere between the other two groups.

But what’s really interesting is how taking an active role in child-rearing seemed to have a particularly chilling effect on a man’s testosterone. Researchers asked fathers about their involvement in the day-to-day duties of child care — activities such as playing with their kids, taking them for walks or on errands, watching television with them and feeding and bathing them — and found that those who reported spending more than three hours a day tending to their kids had the lowest levels of testosterone of all the groups.

“It really seems that the extent to which fathers end up with low testosterone is based on how actively they participate in child care,” says Gettler. “It’s not just that when men become fathers, their testosterone goes down. But when they feed and bathe and play with them, it goes down substantially more. This seems to be an evolved trait, fathers caring for their young.”

Interpreting the finding within an evolutionary framework, it means that men are apparently hard-wired to respond biologically to fatherhood. It’s a fresh perspective, given that the conventional wisdom describes traditional societies as those in which men are hunter-gatherers, while women on the home front forage for berries and care for kids.

In species in which fathers typically pitch in to care for offspring — birds are a prime example — scientists have documented similarly high testosterone levels in mating males that subsequently decline as they help raise their chicks.

It’s simply part of human evolution, says Gettler. Women, throughout the years, have needed help taking care of children; men have answered the call. “Humans wouldn’t have been as successful if fathers weren’t helping,” says Gettler.

Of course, as in any academic discipline, not everyone agrees. Many researchers hew to more traditional evolutionary views that females did the lion’s share of child-rearing — aunts, sisters and grandmothers. And even Gettler is quick to point out that he’s not suggesting fathers were the only people helping Mama.

“It’s an ongoing debate in anthropology,” says Gettler. “But this is suggestive that caring for children is a role fathers were playing in human evolution to the extent that it has become embedded in human male physiology.”

Perhaps cultural dynamics may be at work, says Robin Simon, a sociology professor at Wake Forest University who studies gender and parenthood, among other things. “I’m not familiar with the gendered context of parenting in the Philippines, but primary parenting at least in the U.S. is considered to be a female activity,” says Simon. “Maybe men might find it emasculating.”

For the kid-free Gettler, the research — at least for the time being — is purely theoretical. But due to his newfound knowledge, he’s been encouraging his friends who have had babies recently to get involved with their children. “Of course,” says Gettler, “they are asking me all these questions about their testosterone levels.”

Bonnie Rochman is a reporter at TIME. Find her on Twitter at @brochman. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.